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Back pain is the second most common nerve problem in the United States, after headaches. It is the most common cause of job-related disability, according to the National Institute of Neurological Disorders and Stroke (NINDS).

Back pain is rare in children and teens. It occurs more frequently in people as they age.

Although pain can occur anywhere in the back, it is the low back, or lumbar region, where most people experience pain. The lumbar region includes five vertebrae, along with muscles and ligaments, and supports most of the weight of the upper body. It also does most of the work of bending, stooping, sitting and lifting. Wear and tear and injury can cause discomfort ranging from a mild ache to sudden, sharp, debilitating pain.

Fortunately, most low back pain goes away on its own in a few days to a few weeks. This short-term, or acute, low back pain is usually caused by an injury to the structures of the low back or by arthritis. Pain that lasts three or more months is called chronic back pain. The cause of chronic pain is often difficult to determine.

Symptom, not disease

Low back pain is a symptom of a problem, rather than a disease itself. Conditions that cause it can be the normal aging process, or sprains, strains or spasms in muscles and ligaments, or a bulging (ruptured) disk.

The spine is a column of 30 round bones called vertebrae stacked on top of each other. Each vertebrae has an opening in the center. The vertebral openings are aligned so that they form a tube that runs the length of the spine. This tube contains the spinal cord, the nerves that carry signals and control movement of many parts of the body. Throughout the length of the spine, more than 50 nerves enter and leave the spinal cord through small openings in the sides of the vertebrae. The spaces between the vertebrae are filled with round spongy pads called intervertebral disks. These cushion the bones and absorb shocks to prevent injury to the vertebrae. Ligaments and tendons hold the vertebrae and disks in place and attach the back muscles to the spinal column.

In the aging process, bones of the spine to become thinner and more easily fractured; muscles, tendons and ligaments that support the back become weaker, less flexible and more easily strained; and the disks between the vertebrae become thinner, stiffer and less able to cushion the vertebrae.

Sprains and strains to muscles and ligaments can be caused by improper body mechanics that overstretches them, such as lifting something too heavy, or a sudden awkward movement. Lifting heavy items can also compress the vertebrae and cause a disk to bulge outward, or rupture.

When a portion of an intervertebral disk is moved out of place, or is ruptured, it can bulge into one of the spaces where a nerve enters or leaves the spinal cord. The bulging disk irritates or puts pressure on the nerve, causing a kind of pain called radiculopathy.

Risks

In many cases of back pain, the cause is unknown. These are some of the more common conditions or situations that put a person at risk for back pain:

Improper body mechanics when lifting or bending.

Poor physical conditioning, which causes weak muscles, and inflexible ligaments and tendons. When these areas are weak, they are more easily injured and fail to properly support the spine.

Chronically poor posture when sitting, standing or sleeping.

Standing or sitting for long periods of time, especially if you are jostled by movement while sitting, such as when driving a truck long distances.

Osteoarthritis and other degenerative bone diseases, which can cause a breakdown of the intervertebral disks called spondylosis; bone spurs, and an overgrowth of bone in the vertebrae. This overgrowth narrows both the tube used by the spinal cord and the spaces through which nerves enter and leave the spinal cord. Osteoarthritis can also cause degenerative spondylolisthesis, a slippage of one vertebra over another. Degenerative conditions cause irritation of the nerves, resulting in low back pain, leg pain, muscle spasms and weakness.

Osteoporosis, which is a thinning and weakening of bones throughout the body, not just the vertebrae. This disease makes it easier to fracture bones. Sometimes low back pain may be because of a fracture in the lower part of the pelvis caused by osteoporosis.

Inflammatory forms of arthritis. These include Reiter's disease, ankylosing spondylitis, psoriatic arthritis and the arthritis of inflammatory bowel disease.

Obesity, because extra weight has to be supported by the lower back. Weight carried in front on the spine (a “beer belly,” for example) causes the muscles in the low back to contract into order to hold the belly up. This can lead to muscle spasms, disk degeneration and arthritis in the spine.

Smoking, which reduces blood flow to the lower back. Smoking prevents the disks and muscles of the back from getting enough nutrients; they become weaker and are slower to heal if injured. Smoker’s cough may also cause back pain.

Uncommon causes of back pain include cancer, infection in the vertebrae or spinal column, aortic aneurysm, kidney stones, fibromyalgia, pregnancy, endometriosis and some inherited conditions.

Symptoms

Back pain from a sprain or muscle strain usually has these symptoms and usually can be treated with medication and exercise:

Pain that ranges from an ache to a stabbing pain

Pain in a broad area of the back or on only one side

Pain that is worse with activity and is severe enough that you can’t bend, stand or sit without discomfort

Back pain from a sprain or muscle strain does NOT cause pain or weakness in your legs.

Pain from a ruptured disk depends on the where in the spine the disk is located and the size of the rupture. If the ruptured disk does not press on a nerve, there may be no symptoms and you will not know you have it. If it does irritate a nerve, the symptoms are usually pain, burning, tingling, numbness or weakness in the area the nerve serves.

One common cause of low back pain linked to a ruptured disk is sciatica. Sciatica is caused by irritation of one of the nerves that make up the sciatic nerve. The pain from sciatica is usually described as a sharp, shooting pain that runs from the buttock down the back of the leg, sometimes as far as the foot. It is on one side and may be worse when standing, walking or sitting.

If you have symptoms of a ruptured disk, see your health care provider for an evaluation.

See your health care provider if you have back pain with these symptoms:

A sudden loss of bladder or bowel control, or weakness in a leg; this symptom means you need emergency care, because a ruptured disk may be causing a serious problem with nerves

Pain runs down your leg to your knee or ankle; this may mean a possible ruptured disk

Pain on one side of your back with blood in your urine and burning during urination; this may be a possible kidney stone